Syntactic Phrase Structure

Will Styler - LIGN 6

Today’s Plan

Phrase Structure Rules
Structural Ambiguity

Last time, we talked about constituents

Noun Phrases
Verb Phrases
Prepositional Phrases

… and we realized that the world of syntax isn’t flat

(Neither is the rest of the world, damnit)

Now, let’s think a bit more about those structures

Phrase Structure Rules

We have intuitions about what can be inside a noun phrase in English…

The big, strong rhinoceros
*The big, quickly rhinoceros
*The strong, charged rhinoceros

A verb phrase has constraints in English

I hugged the small kitten
*I small hugged the kitten
*I hugged gently the kitten (???)

And sentences definitely have rules in English

Sam sent Amanda to the store.
*Sent Sam Amanda to the store.
*Amanda Sam to the store sent.

So, English has rules about how phrases are built

… that sounds like a language model to me!

Today, we’ll talk about a very basic approach to modeling syntax

Phrase Structure Rules

Rules that describe the sub-elements which can form a given syntactic constituent
- … and their ordering
Syntax trees are built from phrase structure rules!

Phrase structure rule format

X → Y Z
S → NP VP
- “A sentence is composed of an NP, followed by a VP”

Phrase structure rules are like building blocks of sentences

We build sentences by building chunks using PS rules
- Then combining them into the desired larger forms

How do we build trees?

Building a phrase structure tree

Step 1: Find constituents and think about what kinds of phrases they are
- “Huh, this is a preposition. There’s gotta be a PP around here…”
Step 2: Start stitching constituents together according to our PS rules
Step 3: Check to make sure every branching obeys the phrase structure rules
- “OK, this is Rule 9, now rule 1 to 2, then here’s 11…”
Step 4: A beautiful tree will grow!

A Small Phrase Structure Grammar for English

Noun Phrase Rules

Rule 1) NP -> DET N’

Rule 3) N’ -> N

Rule 1) NP -> DET N’

Rule 3) N’ -> N

… but what if there’s no determiner?

Rule 2) NP -> N’

Rule 3) N’ -> N

Rule 2) NP -> N’

Rule 3) N’ -> N

How do we deal with adjectives?

Rule 4) N’ -> ADJ N’

We can combine Rule 1, Rule 4, and Rule 3…

Rules 1, 3, 4

We can combine Rule 1, Rule 4, and Rule 3…

Rules 1, 3, 4

We can combine Rule 1, Rule 4, and Rule 3…

… Rule 4 is recursive!

The N’ contains another N’!
It can apply as many times as you need!

(… and so on)

What about prepositions? Linguists love prepositions!

## Rule 5: NP -> NP PP ## Rule 6: PP -> P NP

Rules 5 and 6

“Two computational linguists from San Diego”

Now, let’s make a sentence!

Rule 7: S -> NP VP

Rule 8: VP -> V

A very simple sentence

Adding in a preposition (with Rule 10)

But lots of verbs are transitive, and take a noun phrase as the object (Rule 9)

… and we can bring old rules back in!

… and we can bring more old rules back in!

… and we can add on another PP

Is this starting to feel familiar?

Making Syntax Trees is combining small elements to make bigger, fancier trees

Complementizers put sentences within sentences

Words like “that”, “when”, “if”, “while”, “as”, and more

… we can go deeper

… and this combines with everything we’ve been doing …

… and just keeps getting worse …

So that’s how trees are grown

In NLP, trees are usually stored with nested parentheses

(S (NP (NP (DET The) (N’ (ADJ proud) (N’ (N baker)))) (PP (P from) (NP (N’ (N San Bernadino))))) (VP (V loved) (CP (C that) (S (NP (N’ (N people))) (VP (V cheered) (CP (C as) (S (NP (N’ (N Will))) (VP (VP (V ate) (NP (NP (DET the) (N’ (ADJ massive) (N’ (ADJ chocolate) (N’ (N cake))))) (PP (P from)(NP (DET the) (N’ (ADJ new) (N’ (N bakery)))))))(PP (P with) (NP (DET his) (N’ (N wife))))))))))))